The present invention relates to optical fiber interferometric fading, and more particularly to a method of completely eliminating polarization fading in optical fiber interferometers.
A fiber optic interferometer combines the light from two optical paths and causes them to interfere. The interference can be thwarted (i.e., "fade") in several ways. This invention pertains to fading due to polarization drifts. If the states of polarizations of the two arms of the interferometer are co-directional, the interference (sometimes referred to as contrast) is at a maximum and the sensitivity of the interferometer as a sensor or communication channel is greatest. If the polarizations are orthogonal, the two arms do not interfere in the usual configuration, and the sensitivity of the interferometer is zero.
Fiber interferometric sensors attain high sensitivities by exploiting readily achievable long path lengths in the sensing arm of the interferometer. However, the long path lengths also increase the sensor's vulnerability to random environmental perturbations that reduce or "fade" the sensed signal. Interferometers can fade in two ways: (1) the interferometer arms can drift in their relative optical path length (going into and out of quadrature); and (2) the states of polarization (SOP) of the two fields can become orthogonal, which prevents mixing on the photodetector. The first type of fading can be overcome by mechanical length adjustments or passive demodulation schemes, assuming that the polarization fading is not complete. For the second type of fading, an electronically ideal automatic gain control circuit can, in principle, recover signals as the polarization fades, but only at the expense of an arbitrarily large degradation in the signal/noise ratio (SNR). Accordingly, it is desirable to completely eliminate the possibility of total polarization fading and while causing only a minimal (and readily established) reduction in the SNR.
There have been earlier passive methods of treating polarization fading. In one method, nothing is done to control the states of polarization. Instead, as the signal from an interferometer fades, an automatic gain control circuit increases the electrical gain of the amplifier circuitry. Although this method stabilizes the sensitivity of the interferometer, the signal-to-noise ratio (SNR) can degrade without limit. In another method, polarization preserving fiber is used in each arm of the interferometer. Since, however, perfectly polarization preserving fibers and components have not yet been demonstrated, this approach is susceptible to excess phase noise.
In active approaches the states of polarization emerging from the output end of an optical fiber are analyzed and used to form an error signal. A servo device then corrects the polarization state. Such devices are more complicated to use and maintain than passive devices, and achieve good stabilization properties at the expense of complicated servo loops.